Method of making lead storage battery grids



Oct. 20, 1964 s. s'roRcl-IHEIM METHOD 0E MAKING LEAD STORAGE BATTERY GEIDs Filed Aug. 19. 1960 DEL-.IEDE

NQSQQN WHHOAJI NI l United States Patent 3,153,590 METHD F MAKlNG LEAD STORAGE BATTERY GRIDS Samuel Storcllheirn, Forest Hills, NSY., assigner, by direct and mesne assignments, to Alloys Research & Manufacturing Corporation, a corporation of Delaware Filed Aug. 19, 1960, Ser. No. 50,694 Claims. (Ci. 7S226) The present invention is directed to grids for lead storage batteries, and more particularly to a method of manufacture thereof having improved properties.

In the past such grids were made by melting lead and casting it into suitable molds. It was found that such grids, while having excellent corrosion resistance to sulfuric acid, did not have sufiicient strength to withstand the stresses encountered in service and they did not exhibit good resistance to creeping. To overcome these deficiencies it was customary to alloy the lead with antimony, say 7% thereof. While this increased the strength of the grids, the alloying materials were costly and special precautions were necessary in the fabrication thereof. Also, distortion was likely to occur.

The present invention is intended and adapted to overcome the diiiiculties and disadvantages yencountered in the prior art, it being among the objects thereof to devise a process of making lead grids for storage batteries which does not require the use of alloys of lead to produce satisfactory grids.

It is also among the objects of the invention to provide a method of making battery grids, so that the product will have high strength or resistance to stress and also have excellent resistance to corrosion by sulfuric acid.

It is further among the objects of the invention to devise a process which eliminates the necessity of melting and casting the lead, which is simple and does not require complicated or expensive apparatus, and which lends itself to operation on a continuous or semi-continuous basis.

In the practice of the invention, lead in the form of powder is used. Instead of melting the lead, it is pressed in a suitable mold or die under selected conditions, whereby a strong, compact, and corrosion resistant grid is formed. These conditions including the molding at elevated temperatures, say about 300 C., are commercially feasible for the purpose. In the case of lead powder from certain sources, the iilm of oxide on the granules may be too heavy to produce a grid of satisfactory strength, in which case there is provided a pretreatment which reduces the thickness of the oxide film to an extent which permits the formation of excellent grids.

The invention is more fully described in connection with the accompanying drawing constituting a part hereof and in which like reference characters indicate like parts, and in which:

FIG. l is a diagrammatic View of a plant for the manufacture of lead battery grids by the hot pressing of lead powders, in a continuous operation;

FIG. 2 is a diagrammatic view partly in section of a press adapted to carry out the process of the present invention; and

FIG. 3 is an elevational view of the upper die of the press taken along line 3-3 of FIG. 2.

The apparatus includes a melting furnace, into which pig lead 2 is fed. The molten lead is introduced into the powder spinning machine 3 through hopper 4, the machine being suitably heated say to about 100 C. or over as by steam coils 5. Hot powder 6 from spinning machine 3 is transported to hopper 7 from which it is fed at 3 into press 9. From the press, which is preferably of the automatic type, the finished grid is passed at 10 to conveyor belt 11.

As shown in FIG. 2, the press includes a lower platen 12 heated to the desired temperature by electric resistance heating elements 13, and an upper platen 14 also heated by electric heating elements 15. A lower die or mold 16 has a plurality of upstanding projections 17 and an upper die or mold 1S has a plurality of recesses 18 in alinement with and corresponding to projections 17. Lead powder 20 is introduced into lower die 16 and is consolidated to form a dense body by the usual operation of the press.

The basic units required are a powder production unit and a compacting press. The powder production unit shown is a melt spinning apparatus'. Molten lead is poured into ametal container having perforated walls. When the container is rotated at speeds up to 1000 1'.p.m., the molten metal is forced through the perforations and a fine dispersion of lead is obtained. The hot lead powders are conveyed to the hopper which feeds the automatic compacting press provided with heated platens. The pressure, duration and temperature during the pressing cycle is such that material having optimum properties is produced. A typical set of conditions would be a pressure of 20 t.s.i. (tons per square inch), a temperature of about 300 C. and a time under pressure of about 1/zV minute. The completely finishedv grids are ejected from the die onto the conveyor belt and the cycle is repeated automatically.

Referring to FIG. 3, the finished grid may be of the usual conguration consisting of a frame 21 having a terminal 22 and a series of openings 23 which are to be filled with the active storage battery elements.

Impure or waste or scrap lead may be used, as well as lead containing alloying metals. For example, an alternate feed material for the plant may be lead made by chemical reduction processes. Satisfactory grid material has been produced, for example, from lead obtained as a by-product in the manufacture of tetraethyl lead used in motor gasolines. Such lead in the as-received condition is coated with a heavy oxide lm. Thus when this material is hot pressed the resultant plate does not have optimum mechanical properties, the oxide films preventing satisfactory bonding between the particles. A method of pretreating the lead has been devised, however, which permits removal of the surface oxide films thus rendering such powders satisfactory as a feed material for the manufacture of battery grids. The pretreatment of the tetraethyl lead by-product involves heating-the contaminated lead powders in hydrogen at a temperature in excess of the melting point of lead. The oxide skins on the lead particles contain the molten lead and thus prevent them from fusing together into a coherent mass. Thus tetraethyl lay-product lead powders have been treated at 500 C. for 1 hour in hydrogen. Powders so treated remain in the form of discrete particles. The treatment also renders the powder more metallic looking, evidence of the fact that the surface films have been partially removed. Hot pressing of such material yields plate of much higher strength and ductility. The superiority of grids made by the improved process is illustrated by the attached Table I. It summarizes the j fabrication conditions used, the mechanical properties of the pressed discs and their corrosion resistanceto 98 i sulfuric acid. Note that increasing the time under pressure'serves to increase the elongation and strength of the powder fabricated plate while improving the corrosion resistance to sulfuric acid.

:a t C. a.

TABLE I Summary of Fabrication Conditions Used to Prepare Lead Plate from Hot Pressed Powders and the Mechanical Properties and Corrosion Resistance of Suc/z Plate Duration 'Time Ultimate of Expo- Corrosion Type of Lead Powder Pressure, Tempera- Under Tensile Elongation, sure to 9&7?, Rate, Mrls/ t.s.i. ture, C. Pressure, Strength, Percent Sulfurlc Year Minutes p.s.i. Acid,

Hours -140 mesh atomized 10 300 5 3, 900 7.0 118 64 10 300 2 3, 600 3. 4 118 116 10 300 1 2, 700 2.0 118 65 10 300 3, 400 1. 0 118 112 10 325 5 3, 50() 3. 4 119 42 300 2 3, 900 2. 0 43 122 300 2 3, 800 7. 0 42 59 20 300 1 42 126 20 300 M 43 144 Chemically reduced lead 10 310 5 119 180 ..DO 10 325 5 119 182 Applicants spun powder 10 300 5 43 32 Increasing the pressing temperature or pressure also improves the mechanical properties and corrosion resistance as is evident from the mechanical property data for -140 mesh atomized lead pressed at 350 C. and 20 t.s.i. Much higher pressures have been commercially utilized, and this cuts down the temperature and time required for hot pressing.

As compared to prior methods of making such grids, the present process has many advantages, among which are the following:

(l) The invention permits the use of unalloyed lead in battery grids that are normally cast of a lead-antimony alloy containing 7% antimony. Alloyed lead is normally used because pure lead is not strong or creep resistant enough. Battery grids fabricated from pure lead powders have much higher strength than pure cast lead, are more creep resistant and have a higher modulus of elasticity. Thus lead powder fabricated grids manufactured las described in this invention are strong enough for handling and for withstanding the stresses imposed upon them during service. They are also creep resistant so that distortion of the grid does not occur during normal operating conditions at room temperatures or at the higher temperatures encountered during charging.

(2) High grid mechanical strength without the need for using costly alloyed materials. The `strength of plate fabricated by hot pressing various lead powders is highly satisfactory. Powder particle size or more specifically surface area to volume ratio, is particularly important as regards the strength of powder-fabricated plate. Thus as the surface area to volume ratio; i.e., SA/ V, of a powder particle increases, the tensile strength of plate fabric ated from that powder also increases. Perhaps the correlation can best be understood from the fact that the surface of a powder particle is not a perfect mosaic but rather is composed of a variable number of crystalline discontinuities, forcibly bonded to each other. This type of mis-match bonding, in turn, creates numerous localized coherency stresses. Therefore as the .SA/V ratio increases, the number of discontinuities per unit volume increases and the amount of surface area available for bonding also increases. This is in agreement with the oxide dispersion theory in that the greater the surface area per unit volume exhibited by a certain type particle, the greater is its oxide content. When particles are compacted, their oxide films are ruptured. This facilitates the dispersion of oxide inclusions throughout the metal matrix and inhibits the movement of dislocations through the matrix. Strength is thereby increased and very notably at high temperatures.

The oxide content of powder-fabricated plate as such does not adversely affect room temperature ultimate tensile strength but has quite a pronounced effect on the ductility of powder-fabricated plate. Thus, as the oxide content of a given powder product increases, its ductility tends to decrease while its ultimate tensile strength remains virtually unchanged.

Hot consolidation of lead powders also results in a remarkable improvement in high temperature mechanical properties, particularly creep resistance. mesh and -140 mesh powder-fabricated lead strip were subjected to creep testing at 300 F. at a stress level of 300 psi. for a 500 hour period. The data obtained revealed that the majority of deformation occurred during the primary stage of creep. This was true of both the +140 mesh and 140 mesh strip. Minimum creep rates for the powder-fabricated lead strip in the secondary creep stage are given below. Also given is the creep rate of chemical lead. The chemical lead was tested at a temperature of only F., in contrast to 300 F. for the powder-fabricated product.

Type of lead strip: Creep rate, percent/year +140 mesh powder fabricated 0.24- 140 mesh powder fabricated 1.26 Cast-wrought chemical 43.8

The superior elevated temperature properties of the powder-fabricated lead product permits its use in structural applications where the cast-wrought material could not be used.

(3) Excellent corrosion resistance `to sulfuric acid. This is of great importance in Aa lead battery grid because if the grid corrodes excessively, the active material it holds will be lost and the battery will no longer function properly. By hot pressing lead powders, superior corrosion resistance to that of cast alloyed lead has been obtained for unalloyed lead. Thus, battery grids fabricated from hot pressed powders have a longer life. The corrosion rates for cast alloy battery grids and powderfabricated grids are compared as follows.

Corrosion rate in 98% sulfuric Material: acid, mils/ year Conventional cast alloy battery grid 46 Lead granules pressed at 300 C. and 10 t.s.i. 31.8

The excellent corrosion resistance of the powder-fabricated plate is attributed to the oxide lms originally surrounding individual power particles. ISuch films tend to limit corrosion by serving as more or less continuous protective barriers about relatively small volumes of metal in the matrix so that any corrosion which does occur is localized.

(4) Grids may be made at lower cost by the technique described in this invention because the materials used are lower in cost. Conventional battery grids are fabricated of a lead alloy containing 7% antimony. Since lead is much cheaper than antimony, the cost of a battery grid fabricated from pure lead powders is also cheaper. Note that the cost of making lead powder from pure lead is low enough so that there is `still a considerable saving in cost when pure lead powders are used for grid manufacture instead of lead-antimony alloy. Furthermore, less capital investment is required for the fabricating equipment since a conventional automatic press is substituted for a specially designed continuous casting machine.

What is claimed is:

l. A process for fabricating consolidated lead articles such as battery grids and the like comprising the steps of introducing into a mold having the configuration of the ultimate article discrete particles formed of unalloyed lead, the individual particles having a lead oxide lm thereon, and hot pressing said particles in the mold with su'icient heat and pressure applied simultaneously for a predetermined period to rupture the iilms thereon and to consolidate said particles into a dense coherent lead article having a tensile strength amounting to at least 2400 pounds per square inch and having an improved corrosion resistance to acid relative to cast lead alloys.

2. A process, as set forth in claim 1, wherein said particles are hot pressed in said mold in a temperature range of about 250 C. to about 350 C.

3. A process, as set forth in claim 1, wherein said particles are hot pressed in said mold at a pressure of about l0 to 20 tons per square inch.

4. A prooess for fabricating consolidated lead articles such as battery grids and the like, comprising the steps of introducing into a mold having the configuration of the ultimate article discrete particles formed of unalloyed lead, the individual particles having a lead oxide film thereon, and hot-pressing said particles in Said mold for a period of about 1A. minute to ive minutes with a pressure of about 10 to 20 tons per square inch and simultaneously subjecting the particles to heat in a temperature range of 250 C. to 350 C. to rupture the lilms thereon and to consolidate said particles into a dense, coherent lead article having a tensile strength at least equal to 2400 pounds per square inch and improved corrosion resistance to acid as compared to cast lead alloys.

5. A process as set forth in claim 4, wherein said mold has a grid formation to form battery grids.

References Cited in the le of this patent UNITED STATES PATENTS 2,149,596 Gillett et al. Mar. 7, 1939 2,739,526 Hobbs Mar. 27, 1956 2,832,583 Vogt Apr. 29, 1958 2,933,305 Reed et al. Apr. 19, 1960 3,053,654 McKim Sept. 11, 1962 FOREIGN PATENTS 568,248 Canada Dec. 30, 1958 OTHER REFERENCES Goetzel: Treatise on Powder Metallurgy, vol. 1 (1949), pp. 424, 425, 436, 437, and vol. 2 (1950), pp. 719, 720. 

1. A PROCESS FOR FABRICATING CONSOLIDATED LEAD ARTICLES SUCH AS BATTERY GRIDS AND THE LIKE COMPRISING THE STEPS OF INTRODUCING INTO A MOLD HAVING THE CONFIGURATION OF THE ULTIMATE ARTICLE DISCRETE PARTICLES FORMED OF UNALLOYED LEAD, THE INDIVIDUAL PARTICLES HAVING A LEAS OXIDE FILM THEREON, AND HOT PRESSING SAID PARTICLES IN THE MOLD WITH SUFFICIENT HEAT AND PRESSURE APPLIED SIMULTANEIOUSLY FOR A PREDETERMINED PERIOD TO RUPTUR THE FILMS THEREON AND TO CONSOLIDATE SAID PARTICLES INTO A DENSE COHERENT LEAD ARTICLE HAVING A TENSILE STRENGTH AMOUNT TO AT LEAST 2400 POUNDS PER SQUARE INCH AND HAVING AN IMPROVED CORROSION RESISTANCE TO ACID RELATIVE TO CAST LEAS ALLOYS. 